In a mobile communications system, a base station transmits an amplified high frequency multi-carrier signal having a plurality of carriers separated from each other by different frequency bands and respectively modulated in proper ways. Since, however, an amplification apparatus with a poor linearity produces various kinds of distortion, e.g., inter-modulation distortion, the amplification apparatus for use in the amplification of the high frequency multi-carrier signal is required to exhibit good linearity characteristics across the whole frequency band the multi-carrier signal belongs to.
One of known various methods for realizing an ultra low distortion amplification apparatus suitable for the amplification of, e.g., a multi-carrier signal is to employ a feed forward (referred to as “FF” hereinafter) distortion compensation technique.
The conventional FF amplification technique is implemented by a main line, for amplifying an input multi-carrier signal by a main amplifier and outputting the amplified multi-carrier signal, and a FF loop. The FF loop includes a distortion detection loop for detecting a distortion component generated by the main amplifier from the amplified multi-carrier signal and a distortion compensation loop for canceling the distortion component from the amplified multi-carrier signal by using the distortion component detected in the distortion detection loop.
A conventional non-linear distortion compensation amplification apparatus using such FF amplification technique is disclosed in, for example, Japanese Patent Laid-Open Publication No. 1995-303050 or 1996-307161. The basic configuration and operation of such conventional non-linear distortion compensation amplification apparatus are described with reference to FIG. 3, which includes an input terminal 1, dividers 2, 7 and 12, variable attenuators 3 and 9, variable phase shifters 4 and 10, a main amplifier 5, coaxial delay lines 6 and 8, an auxiliary amplifier 11, an output terminal 13, a control unit 14 and a control signal generation circuit 15.
Referring to FIG. 3, a signal path from the input terminal 1 to the output terminal 13 via the divider 2, the main amplifier 5, the divider 7, the coaxial delay line 8 and the divider 12 forms a main line. On the main line, an input signal (a multi-carrier signal in this embodiment) from the input terminal 1 is divided into a main divided signal and a subsidiary signal by the divider 2. The main divided signal is provided to the main amplifier 5 via the variable attenuator 3 and the variable phase shifter 4 and is high-frequency-amplified by the main amplifier 5. The amplified multi-carrier signal (signal A) is divided into a primary divided signal E and a secondary divided signal (signal C) by the divider 7. The primary divided signal E is delayed by the coaxial delay line 8 by a first predetermined amount and then is outputted to the output terminal 13 by passing through the divider 12.
In case where the main amplifier 5 does not exhibit a good linearity for example, there occurs on such main line inter-modulation of a carrier in the multi-carrier signal. Accordingly, various distortions including, e.g., inter-modulation distortion caused by the inter-modulation of the multi-carrier of the input signal are produced and then mixed into the multi-carrier signal.
In order to remove such distortion, a distortion detection loop L1 and a distortion compensation loop L2 of the FF loop are installed in such non-linear distortion compensation amplification apparatus. The distortion detection loop L1 detects a distortion component, which is generated by the main amplifier 5 and then is mixed into the multi-carrier signal, and the distortion compensation loop L2 removes the distortion component mixed into the multi-carrier signal by using such distortion component detected from the distortion detection loop L1.
The distortion detection loop L1 is configured by the variable attenuator 3, the variable phase shifter 4, the main amplifier 5, the coaxial delay line 6 and the dividers 2 and 7. In the distortion detection loop L1 of such configuration, the multi-carrier signal inputted from the input terminal 1 is provided to the divider 2 and divided into the main divided and the subsidiary signal as mentioned above. The main divided signal is provided to the main line and the subsidiary signal is delayed in the coaxial delay line 6 by a second predetermined amount and then is provided to the divider 7 as a divided signal B.
The divider 7 functions to divide the output signal A of the main amplifier 5 into the primary divided signal E and the secondary divided signal C and provides the primary divided signal E of the output signal A to the coaxial delay line 8 on the main line. The divider 7 further serves to subtract the divided signal B of the coaxial delay line 6 from the subordinate signal of the output signal A provided by the main amplifier 5 to provide a difference signal D. The difference signal D obtained in the subtraction process is provided to the variable attenuator 9 of the distortion compensation loop L2.
The amount of delay in the coaxial delay line 6 is set to be equal to the total amount of delay of the variable attenuator 3, the variable phase shifter 4 and the main amplifier 5. The amount of attenuation of the variable attenuator 3 is set in such a manner that an amplitude of the secondary divided signal C separated from the output signal A by the divider 7 is identical to that of the divided signal B of the coaxial delay line 6. Further, the amount of phase shift of the variable phase shifter 4 is set such that a phase of the secondary divided signal C coincides with that of the divided signal B.
Therefore, the difference signal D outputted from the divider 7 corresponds to such distortion component as inter-modulation distortion generated by the main amplifier 5. The amount of attenuation of the variable attenuator 3 and the amount of phase shift of the variable chase shifter 4 are respectively controlled by control signals G1 and θ1 generated from the control signal generation circuit 15 of the control unit 14 in such a manner that the difference signal D includes only the distortion component.
The distortion compensation loop L2 is configured by the coaxial delay line 8, the variable attenuator 9, the variable phase shifter 10, the auxiliary amplifier 11 and the dividers 7 and 12. In the distortion compensation loop L2 of such configuration, the primary divided signal E, a multi-carrier signal, that is, the main signal of the output signal A from the main amplifier 5 obtained by the divider 7, is delayed in the coaxial delay line 8 by the first predetermined amount and then is provided to the divider 12.
Further, the distortion component D obtained by the divider 7 is provided to the auxiliary amplifier 11 via the variable attenuator 9 and the variable phase shifter 10. A distortion component F, which corresponds to the distortion component D amplified by the auxiliary amplifier 11, is provided to the divider 12. The divider 12 has a subtraction function, to thereby subtract the distortion component F provided by the auxiliary amplifier 11 from the multi-carrier signal E of the coaxial delay line 8. Accordingly, a multi-carrier signal G free from the distortion component generated by the main amplifier 5 is obtained and then outputted to the output terminal 13.
Herein, the amount of delay on the coaxial delay line 8 is set to be equal to the total amount of delay of the variable attenuator 9, the variable phase shifter 10 and the auxiliary amplifier 11. The amount of attenuation of the variable attenuator 9 is set in a fashion that the amplitude of the distortion component mixed into the multi-carrier signal E outputted from the divider 7 is identical to that of the distortion component F of the auxiliary amplifier 11. Further, the amount of phase shift of the variable phase shifter 10 is set in such a manner that a phase of the distortion component F of the auxiliary amplifier 11 is inverted with respect to that of the distortion component mixed into the multi-carrier signal.
Therefore, when the described setting amounts are precisely set, the multi-carrier signal G, in which the distortion component is cancelled out, is obtained at the divider 12. The amount of attenuation of the variable attenuator 9 and that of phase shift of the variable phase shifter 10 are respectively controlled by control signals G2 and 02 generated from the control signal generation circuit 15 of the control unit 14, so that the cancellation of the distortion component can be appropriately achieved.
In the Japanese Patent Laid-Open Publication No. 1996-307161, the amounts of delay of the variable attenuators 3 and 9 and phase shift of the variable phase shifters 4 and 10 are set when the power is applied in such FF amplification apparatus. For this, there are provided a monitor for monitoring the output signal G of the divider 12, a temperature detector, and a memory table having control signals specifying the amounts of attenuation of the variable attenuators 3 and 9 and those of phase shift of the variable phase shifters 4 and 10 with respect to detected output levels of the monitor and the detected temperatures of the temperature detector. Control signals corresponding to a detected output level and a detected temperature are read from the memory table to thereby control the amounts of attenuation of the variable attenuators 3 and 9 and those of phase shift of the variable phase shifters 4 and 10.
Accordingly, the distortion component can be suppressed to be minimal at the beginning of the power application. Further, in case where a gain fluctuation is caused by a fault of the main amplifier 5 or the auxiliary amplifier 11 during a normal operation, a discrepancy is detected between an actual control signal being currently used and a normal control signal specified in the memory table corresponding to the currently detected output level and temperature. An alarm signal is issued based on the detection of the discrepancy, to thereby announce the faulted state of the apparatus.
Further, the conventional technique disclosed in Japanese Patent Laid-Open Publication No. 1995-303050 sets the amounts of attenuation of the variable attenuators 3 and 9 and phase shift of the variable phase shifters 4 and 10 by using pilot signals, wherein a variable attenuator and a variable phase shifter in combination are referred to as a vector modulator.
That is, in FIG. 3, a first pilot signal is inputted from the input terminal 1 and a second pilot signal is inputted to the divider 7 at the output end of the main amplifier 5. The first pilot signal is extracted from the difference signal D from the divider 7 to check the distortion detection loop L1 and the vector modulator, i.e., the variable attenuator 3 and the variable phase shifter 4, of the distortion detection loop L1 is controlled in such a manner that the difference signal D is free from the first pilot signal.
Further, the second pilot signal is checked whether it is included in the output signal G of the divider 12 and a setting value of the vector modulator, i.e., the variable attenuator 9 and the variable phase shifter 10, of the distortion compensation loop L2 is controlled such that the second pilot signal is prevented from being included in the output G.
In the conventional technique disclosed in Japanese Patent Laid-Open Publication No. 1995-303050, the detection levels of the first and the second pilot signal can be almost zero if the proper compensation is carried out. However, if the system is in a faulted state, e.g., if the main amplifier 5 is out of order, the detected levels do not become and may exceed a predetermined level. Then the system informs an operator or a maintenance personnel on such case and cuts off power at the same time.
As described above, if the main amplifier 5 normally operates, the conventional FF amplification apparatus as shown in FIG. 3 can set the amount of attenuation of the variable attenuator 3 and that of phase shift of the variable phase shifter 4 in such a fashion that the difference signal D outputted from the divider 7 contains distortion component only. However, if a gain of the main amplifier 5 is reduced, e.g., by a fault of a component of the main amplifier 5, the gain reduction of the main amplifier 5 may not be compensated by the amount of attenuation of the variable attenuator 3. That is, the gain of the main amplifier 5 may be reduced to exceed an adjustable range of attenuation of the variable attenuator 3.
In such case, the conventional technique disclosed in Japanese Patent Laid-Open Publication No. 1996-307161 posts an alarm on detecting such faulted state, while the conventional technique disclosed in Japanese Patent Laid-Open Publication No. 1995-303050 informs the case and simultaneously cuts off power.
However, if cutting off the power upon detecting the malfunction of the main amplifier, a mobile communications base station and a relay station using such FF-amplification apparatus are disenabled, to thereby unable to perform mobile communications. Further, if merely posting an alarm while maintaining the operation of the FF amplification apparatus as it is, as disclosed in Japanese Patent Laid-Open Publication No. 1996-307161, the amplitude of the secondary divided signal C of the output signal A of the main amplifier 5 cannot be make to coincide with that of the divided signal B of the coaxial delay line 6. Accordingly, the difference signal D outputted from the divider 7 will include carriers of the multi-carrier signal as well as distortion components. Further, since the amplitude of the distortion component in the output signal F of the auxiliary amplifier 11 cannot be equal to that of the distortion component in the multi-carrier signal E on the coaxial delay line 8, the distortion component remains in the output signal G from the divider 12.